Bijou All Tube Futterman Headphone Amplifier - Page 145

Oooh........nice knobs. ;-)

Yep, adamus is right. This does not show up in Spice. I just started reducing the value of the stopper until it oscillated. It was not hard to detect as the amp pretty much stopped working and I could hear the tubes singing.

Generally there are two knobs, the open and close loop gain behaviors. Adjusted as I mentioned.

In the Bijou case there is another effect that matters that is essentially equivalent to the Cdom cap on the VAS stage.

In the Bijou, the only gain comes from the first stage. The Miller capacitance between the grid and plate is, essentially, the Cgp mulitplied by the gain of the stage which is about 25. I don't recall the Cgp for 6922, but if it's 3pf then the Milller capacitance is 75pf. Now the Miller capacitance is identical to the Cdom cap that is introduced to reduce the HF open loop gain. So we've already got a sizeable Cdom cap built into the first stage.

In addition to this the grid stopper resistor and the Miller capacitance act like a low pass filter. The bigger the grid stopper the lower the corner frequency. With no NFB this would just change the HF rolloff, but with feedback it affects the closed loop gain and the phasing. Reducing the grid stopper resistor increases the rolloff frequency giving the amp more gain at high frequencies. In the case of the Bijou, with the phase shifts introduced by the phase splitter and output stages, increasing the closed loop rolloff frequency by lowering the value of the stopper eventually causes the amp to oscillate. Increasing the grid stopper prevents this closed loop oscillation by doing just what a grid stopper is supposed to do.

10k is sufficiently high to prevent oscillation under all conditions while permitting a nice range of NFB with a reasonable pot.

BTW, sand devices have Miller capacitance too. Mosfets, because of their Cgd can have quite a lot.

That Sansui receiver is a beauty. Is that an old picture from when it was new, when, back in the 1970s or has it been kept so beautifully? Four level meters - Quadrophonic?

My Bijou kit will be arriving from Glass Jar in a couple of days, but I still need to order resistors for Regal's Mod. Does anyone have a Mouser (or wherever) parts list for these as well as the variants for R2? Need 10k@1W; 30K@2W; 487, 180, and 1.3k @ 1/4W 1% metal film. Can only find metal oxide for the 2W R3. Is that ok? I was looking through the Mouser catalog and the higher watt resistors needed are mostly longer than the 10-12 mm lead spacing called for in runeight's parts list. Is that a big deal? Also, I'm not sure what to look for in sockets for R2. Thanks, Vic

pabbi1 are you using the standard 2CP-2511 or PCX's 2CP-2508 Alps A taper? And how does it compare to an Alps or cheap stepped attenuator?

I bought a set of Bijou boards in the original group buy and have been slowly collecting parts as finances have allowed. Now have just about all the parts to start building and was planning to use a spare Alps pot I have. Is it worth the extra to get a better pot?

Well, 'worth it' is different for everyone, and many will tell you the rk27 is just dandy. Have a gander at this thread, where the illuminati all weigh in. I have had varying degrees of success with eBay steppers, though the NON-smd have given me the best results. I did indeed get the P2508, where many others like the straight P2511, preferring to save the cash.

Vic, since I didn't do the Regal mod (maybe I should do a retrofit), but generally, if you cannot find resistors at Mouser, there are several honey holes to try. First is PartsConnexion, second is Soniccraft, my third choice (takes a little patience) is Nebraska Surplus Salas, and, of course, good old eBay has several Asian parts stores, and a few US guys. Michael Percy (Percyaudio) also has anything you need, but I have never ordered anything from him.

There are others, but far be it from me to be able to remember them all...

metal oxide is fine.

for socketing, you need SIP sockets:
310-13-120-41-001000

i think i had to "tombstone" only R7 and R8 on the PS board. i think i used Vishay RN55 everywhere else without lead spacing issue.

Thanks for your help Alex. So, given the worse case scenario (untrimmed DC offsetS for example) what sort of voltage ratings are appropriate for the compensation capacitors from grid to plate as well as the capacitors in the feedback loop?

Do you have a particular circuit in mind? At his point it would help to see it. PM me if that's prefereable. Or email me directly if you have my email address.

As I recall, R3 is the one large resistor on the board and is given a little extra space. I used this 30k 2W 5% for my R3 and it fits just fine.

If that Sansui is like most analog radio receivers, those four meters indicate left level, right level, tuning accuracy and radio signal strength.

That Sansui receiver was something for sale on Ebay a while ago. I think it went for $350.00

Here are a couple of pics with knobs temporarily mounted into the front panel. They are going to lose 2mm off of their diameter and 4mm off of their thickness. Fairly easy to do mounted in the drill press.

Amphead, how's the casework coming?

Thanks Pabbi, fishski, and Chobint. I put in an order to Mouser for resistors for Regal's Mod and for tube rolling, Wima's for C6, jacks, power inlet, and some odds and ends. Also ordered a pair of 6DJ8. Next up is wire, transformer, and decision on case.

I would like to build the amp boards upside down under the top plate and air wire the tube sockets. TimmyMac soldered straight to the boards for his air wire. It looks like Ferrari soldered sockets to the boards and wired from the sockets - does that make for a less positive contact?

If the PS board is upside down will the FETs get too hot? Can I air wire those too and mount them on top with the tubes (and tranny) like on my Starving Student Millett?




or will all of that air wiring likely result in hum or signal degradation problems regardless of how carefully routed? If it is a problem, would shielded wire fix it?

Thanks,
Vic

I am considering building the Bijou but I am concerned about driving low Z phones (Denon d5K). I have read multiple posts describing some ideas for mods but I have not read any opinions about how the phones sound. I would appreciate if anyone would share their impressions and amp configuration (tube/caps/etc)?

You will be driving 25 ohm phones, therefore it requires a fair amount of negative feedback. It can be done successfully, and I would recommend installing the NFB pot, to allow you to dial in the sound that best suits the Denon AH-D5000 phones. Obligato, Mundorf silver/oil, Auricap are some recommended films for bypassing the output caps, which will probably be lytics, due to the cost of large films. If you decided to give it a go, welcome to the Bijou club!

Edit: I have resized one of my volume knobs and polished it to a near mirror finish with automotive finishing polish. Pics when both are done and mounted.

I hate turning off my Bijou because I can never predict when I will want to listen to it and don't like waiting for the warm up.

This is expensive when using Aperex 6DJ8 tubes. Is there an easy way to wire the standard transformer to power the 12V heated Aperex 8416's ?

if by standard you mean hammond with a single 6.3v ct secondary then i dont think so. Its possible with the R core as it has 2 6.3v secs.

a very easy way would be to use a small 12v tranformer. Unregulated it will put out about 12.6v anyway if its a bit over capacity.